1. Field of the Invention
The invention relates to a communication apparatus, particularly to a communication apparatus which changes the measuring method of the communication quality according to the communication conditions.
2. Description of the Related Art
Recently, cdma2000 1x EV-DO (hereinafter referred to as “EV-DO”) has been developed as a next-generation high-rate radio communication system. The EV-DO is a version of specializing cdma2000 1x provided by expanding cdma2000 to the third-generation system for data communications and increasing the transmission rate. “EV” means Evolution, and “DO” means Data Optimized.
The EV-DO system is approximately the same as the cdma2000 1x system in the configuration of a radio interface of a reverse channel from a radio communication terminal to a base station. As for the configuration of the radio interface of a forward channel from a base station to a radio communication terminal, whereas the bandwidth of 1.23 MHz is the same with that of the cdma2000 1x system, the modulation method and the multiplexing method are much different from those of the cdma2000 1x system. As for the modulation method, whereas QPSK and HPSK are used in the cdma2000 1x system, QPSK, 8-PSK, or 16QAM is selectively used in the EV-DO system according to a reception state of a forward channel of a radio communication terminal. As a result, a high transmission rate with low error resistance is used when the reception state is good, and a low transmission rate with high error resistance is used when the reception state is bad.
As for the multiplexing method for enabling multiple communications from one base station to a plurality of radio communication terminals, TDMA (Time Division Multiple Access) is employed in which communications with a plurality of radio communication terminals are performed in such a manner that time is divided in units of 1/600 second, a communication is performed with one radio communication terminal in each unit time, and the radio communication terminal to be communicated is switched every unit time, rather than CDMA (Code Division Multiple Access) which is employed in the cdmaOne system and the cdma2000 1x system.
A radio communication terminal measures a carrier-to-interference power ratio (hereinafter abbreviated as CIR) of a pilot signal as an index of a reception state of a forward channel from a base station to be communicated, predicts a reception state during the next reception time slot on the basis of a variation of the CIR, and notify “a maximum transmission rate which enables to receive with a error ratio that is lower than a predetermined rate”, which is expected from the predicted reception state to the base station as data rate control bits (hereinafter referred to as DRC) predetermined. The predetermined error rate is usually set to about 1% though it depends on the system design. The base station receives the DRCs from a plurality of radio communication terminals, and a scheduler function in the base station determines with which radio communication terminal is to communicate in each division unit time. Basically, as high a transmission rate as possible is decided on the basis of a DRC sent from each radio communication terminal and is used for a communication with it.
With the above configuration, the EV-DO system enables a maximum transmission rate of 2.4 Mbps (mega-bits per second) per sector in a forward channel. This transmission rate is the sum of amounts of data communications from one base station to a plurality of radio communication terminals in one frequency band and in one of a plurality of sectors (usually, a plurality of sectors exist). The transmission rate increases if a plurality of frequency bands are used.
JP-A-2002-300644 is referred to as a related art.
In the case of including a plurality of mobile communication paths as the EV-DO system, the convenience for users is enhanced if it is possible to seamlessly switch among a plurality of the mobile communication paths, and for a user to perform communication through any of the mobile communication paths. In order to implement the switching, each radio quality information of the plurality of mobile communication paths are generally monitored by sampling at regular intervals. In the mobile communication path during data communication, various radio information are capable of being sampled at regular intervals. On the other hand, in the mobile communication path not being selected as the communication path, or even being selected but having no data to be communicated, the mobile communication terminal may be in the sleep mode for the purpose of battery saving. Therefore, the radio quality of the mobile communication path should be monitored by a different method from that in data communication. In the sleep mode, the RF circuit is operated for a short period in a given cycle, for example, for 100 ms (millisecond), once per 5 seconds, the mobile communication terminal carries out the tracking of a base station and the check of the incoming data.
In such a case, generally, the mobile communication terminal does not distinctly detect that it has shifted to the sleep mode. Therefore, since the acquired radio quality information is not classified as those in communication or those in the sleep mode, the reliability of the acquired radio quality information is not insured. As a countermeasure thereagainst, an echo back command is issued even when there has been no data to be communicated in order to prevent the mobile communication terminal from shifting to the sleep mode. However, unless the mobile communication terminal shifts to the sleep mode, the consumption of the battery is increased.